diff --git a/mem0/memory/main.py b/mem0/memory/main.py
index 30b4fa7bc..45511a973 100644
--- a/mem0/memory/main.py
+++ b/mem0/memory/main.py
@@ -1842,8 +1842,9 @@ class Memory(MemoryBase):
         try:
             existing_memory = self.vector_store.get(vector_id=memory_id)
         except Exception:
+            # Backing-store failure, not a bad memory_id: re-raise the original so the REST layer maps it to 5xx, not 4xx.
             logger.error(f"Error getting memory with ID {memory_id} during update.")
-            raise ValueError(f"Error getting memory with ID {memory_id}. Please provide a valid 'memory_id'")
+            raise
 
         if existing_memory is None:
             raise ValueError(f"Memory with id {memory_id} not found. Please provide a valid 'memory_id'")
@@ -3406,8 +3407,9 @@ class AsyncMemory(MemoryBase):
         try:
             existing_memory = await asyncio.to_thread(self.vector_store.get, vector_id=memory_id)
         except Exception:
+            # Backing-store failure, not a bad memory_id: re-raise the original so the REST layer maps it to 5xx, not 4xx.
             logger.error(f"Error getting memory with ID {memory_id} during update.")
-            raise ValueError(f"Error getting memory with ID {memory_id}. Please provide a valid 'memory_id'")
+            raise
 
         if existing_memory is None:
             raise ValueError(f"Memory with id {memory_id} not found. Please provide a valid 'memory_id'")
diff --git a/server/main.py b/server/main.py
index 7476698a1..d10a9d67c 100644
--- a/server/main.py
+++ b/server/main.py
@@ -19,6 +19,7 @@ from errors import (
 from fastapi import Depends, FastAPI, HTTPException, Request
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse, RedirectResponse
+from mem0.exceptions import ValidationError as Mem0ValidationError
 from models import RequestLog, User
 from pydantic import BaseModel, Field
 from rate_limit import limiter
@@ -206,6 +207,14 @@ class GenerateInstructionsRequest(BaseModel):
     use_case: str = Field(..., description="Description of what the user will use Mem0 for.")
 
 
+def _client_error(exc: Exception) -> HTTPException:
+    """Map core validation / not-found errors to 4xx so clients can tell a bad
+    request from an upstream outage. 'not found' is a 404, everything else a 400."""
+    detail = str(exc)
+    status_code = 404 if isinstance(exc, ValueError) and "not found" in detail.lower() else 400
+    return HTTPException(status_code=status_code, detail=detail)
+
+
 def _redact_config(value: Any, key: str | None = None) -> Any:
     if isinstance(value, dict):
         return {item_key: _redact_config(item_value, item_key) for item_key, item_value in value.items()}
@@ -363,6 +372,8 @@ def add_memory(memory_create: MemoryCreate, _auth=Depends(verify_auth)):
         if response.get("results"):
             telemetry.log_dashboard_nudge_once(DASHBOARD_URL)
         return JSONResponse(content=response)
+    except (ValueError, Mem0ValidationError) as e:
+        raise _client_error(e)
     except Exception:
         raise upstream_error()
 
@@ -466,6 +477,8 @@ def update_memory(memory_id: str, updated_memory: MemoryUpdate, _auth=Depends(ve
         return get_memory_instance().update(
             memory_id=memory_id, data=updated_memory.text, metadata=updated_memory.metadata
         )
+    except (ValueError, Mem0ValidationError) as e:
+        raise _client_error(e)
     except Exception:
         raise upstream_error()
 
@@ -485,6 +498,8 @@ def delete_memory(memory_id: str, _auth=Depends(verify_auth)):
     try:
         get_memory_instance().delete(memory_id=memory_id)
         return MessageResponse(message="Memory deleted successfully")
+    except (ValueError, Mem0ValidationError) as e:
+        raise _client_error(e)
     except Exception:
         raise upstream_error()
 
diff --git a/tests/test_memory.py b/tests/test_memory.py
index 71a86d438..da00953ef 100644
--- a/tests/test_memory.py
+++ b/tests/test_memory.py
@@ -536,6 +536,59 @@ async def test_async_update_nonexistent_memory_raises_error(mock_sqlite, mock_ll
     mock_vector_store.update.assert_not_called()
 
 
+@patch('mem0.utils.factory.EmbedderFactory.create')
+@patch('mem0.utils.factory.VectorStoreFactory.create')
+@patch('mem0.utils.factory.LlmFactory.create')
+@patch('mem0.memory.storage.SQLiteManager')
+def test_update_propagates_vector_store_failure(mock_sqlite, mock_llm_factory, mock_vector_factory, mock_embedder_factory):
+    """A backing-store failure while fetching the memory during update must
+    surface as the original error, not be masked as a 'provide a valid
+    memory_id' ValueError. The REST layer relies on this so an outage maps to
+    5xx instead of a misleading 4xx."""
+    mock_embedder_factory.return_value = MagicMock()
+    mock_vector_store = MagicMock()
+    mock_vector_factory.return_value = mock_vector_store
+    mock_llm_factory.return_value = MagicMock()
+    mock_sqlite.return_value = MagicMock()
+
+    from mem0.memory.main import Memory as MemoryClass
+    config = MemoryConfig()
+    memory = MemoryClass(config)
+
+    mock_vector_store.get.side_effect = ConnectionError("vector store unreachable")
+
+    with pytest.raises(ConnectionError, match="vector store unreachable"):
+        memory._update_memory("mem-1", "new data", {"new data": [0.1, 0.2]})
+
+    mock_vector_store.update.assert_not_called()
+
+
+@pytest.mark.asyncio
+@patch('mem0.utils.factory.EmbedderFactory.create')
+@patch('mem0.utils.factory.VectorStoreFactory.create')
+@patch('mem0.utils.factory.LlmFactory.create')
+@patch('mem0.memory.storage.SQLiteManager')
+async def test_async_update_propagates_vector_store_failure(mock_sqlite, mock_llm_factory, mock_vector_factory, mock_embedder_factory):
+    """Async twin: a backing-store failure during update re-raises the original
+    error instead of masking it as a ValueError."""
+    mock_embedder_factory.return_value = MagicMock()
+    mock_vector_store = MagicMock()
+    mock_vector_factory.return_value = mock_vector_store
+    mock_llm_factory.return_value = MagicMock()
+    mock_sqlite.return_value = MagicMock()
+
+    from mem0.memory.main import AsyncMemory
+    config = MemoryConfig()
+    memory = AsyncMemory(config)
+
+    mock_vector_store.get.side_effect = ConnectionError("vector store unreachable")
+
+    with pytest.raises(ConnectionError, match="vector store unreachable"):
+        await memory._update_memory("mem-1", "new data", {"new data": [0.1, 0.2]})
+
+    mock_vector_store.update.assert_not_called()
+
+
 @patch('mem0.utils.factory.EmbedderFactory.create')
 @patch('mem0.utils.factory.VectorStoreFactory.create')
 @patch('mem0.utils.factory.LlmFactory.create')
diff --git a/tests/test_server_params.py b/tests/test_server_params.py
index 6ee58de69..f5700ad4c 100644
--- a/tests/test_server_params.py
+++ b/tests/test_server_params.py
@@ -12,6 +12,8 @@ from unittest.mock import MagicMock, patch
 
 import pytest
 
+from mem0.exceptions import ValidationError as Mem0ValidationError
+
 pytest.importorskip("fastapi", reason="fastapi not installed")
 
 from fastapi.testclient import TestClient
@@ -701,3 +703,44 @@ class TestSearchValidationErrors:
         )
         resp = client.post("/search", json={"query": "food"})
         assert resp.status_code == 400
+
+
+# ===========================================================================
+# add / update / delete: map core errors to 4xx instead of 502
+# ===========================================================================
+
+class TestWriteHandlerErrorMapping:
+    """ValueError("... not found") -> 404, other ValueError / Mem0ValidationError
+    -> 400. A real outage still surfaces as 502 via upstream_error()."""
+
+    def test_update_not_found_returns_404(self, client, mock_memory):
+        mock_memory.update.side_effect = ValueError("Memory with id mem-1 not found")
+        resp = client.put("/memories/mem-1", json={"text": "new"})
+        assert resp.status_code == 404
+        assert "not found" in resp.json()["detail"]
+
+    def test_delete_not_found_returns_404(self, client, mock_memory):
+        mock_memory.delete.side_effect = ValueError("Memory with id mem-1 not found")
+        resp = client.delete("/memories/mem-1")
+        assert resp.status_code == 404
+
+    def test_update_other_value_error_returns_400(self, client, mock_memory):
+        mock_memory.update.side_effect = ValueError("data must be a non-empty string")
+        resp = client.put("/memories/mem-1", json={"text": "new"})
+        assert resp.status_code == 400
+
+    def test_add_validation_error_returns_400(self, client, mock_memory):
+        mock_memory.add.side_effect = Mem0ValidationError(
+            message="messages must be str, dict, or list[dict]", error_code="VALIDATION_003"
+        )
+        resp = client.post("/memories", json={
+            "messages": [{"role": "user", "content": "hi"}], "user_id": "u1",
+        })
+        assert resp.status_code == 400
+
+    def test_add_real_outage_still_returns_502(self, client, mock_memory):
+        mock_memory.add.side_effect = RuntimeError("vector store unreachable")
+        resp = client.post("/memories", json={
+            "messages": [{"role": "user", "content": "hi"}], "user_id": "u1",
+        })
+        assert resp.status_code == 502